Mangjul Ilrang has one of the rarest surnames in South Korea, one that might even be considered hazardous in this country, where anti-Japanese sentiment still runs deep. Only nine other South Koreans share it: his own four children and five grandchildren.

“I tell my children to tell people that it’s a Japanese name and be proud of it,” Mr. Mangjul, 67, said in an interview at his home.

How Mr. Mangjul ended up with his unusual name is a story of how he made peace with both Japan, his father’s country, and South Korea, his mother’s. It tells one man’s struggle to salvage an identity amid the turmoil that engulfed the two nations for much of the last century.

In South Korea, just 20 surnames — including Kim, Lee and Park — account for 65 percent of the population. An unconventional name, particularly one that reflects Japanese roots, invites scrutiny, not always friendly.

“Mangjul Ilrang” sounds neither Korean to Koreans nor Japanese to Japanese. It is written in the Chinese characters sometimes used for names in both countries. They are pronounced Ichiro Amigiri in Japanese, which was his original name, but Mangjul Ilrang in Korean, which is how he says it now....